1. Field of the Invention
The present invention relates to a lighting device and a manufacturing method thereof. More specifically, the present invention relates to a light-emitting device using a nitride semiconductor emitting ultraviolet light and a manufacturing method thereof.
2. Description of the Background Art
A GaN-based compound semiconductor functions as a blue TED (Light Emitting Diode) or an ultraviolet LED because of its large band gap, and is often used as an excitation light source of a white LED. Suggestions such as the following have been made for improvement of performance of the GaN-based LED which emits ultraviolet light having a short wavelength.
(d1) Using a SiC substrate and an InAlGaN layer as a light-emitting layer, and adjusting a composition of In, for example, in the InAlGaN layer to increase efficiency of light emission within an ultraviolet region of 360 nm or shorter wavelengths (Japanese Patent Laying-Open No. 2001-237455).
(d2) Using as a light-emitting layer a single-layer quantum well structure formed with Al0.1Ga0.9N layer/Al0.4Ga0.6N layer formed on a GaN substrate to increase brightness (T. Nishida, H. Saito, N. Kobayashi; Appl. Phys. Lett., Vol. 79 (2001) 711).
The above-described ultraviolet light-emitting device, however, has low light emission efficiency, and the light emission efficiency decreases because of heat production when a large current is passed for use in illumination. A reason for the low light emission efficiency of the aforementioned ultraviolet light-emitting device is its high dislocation density in the substrate and the light-emitting layer, which dislocation works as a non-radiative center. In particular, when a sapphire substrate is used, it does not dissipate heat efficiently and there is a strong tendency of light emission efficiency not to increase linearly in proportion to an input, but to be saturated halfway.